This invention relates to a method of removing contaminants from the surface of an article and, more particularly, a method of removing contaminants from the internal components of an air-breathing machine such as an aircraft gas turbine engine. The invention finds specific application in the removal of contaminants from vanes and blades associated with the compressor of an aircraft gas turbine engine of the high by-pass fan type.
In a high by-pass fan type gas turbine engine, a compressor supplies air under pressure to a combustion chamber in which fuel is mixed with the pressurized air and the mixture burned. The hot products of combustion are passed sequentially through a pair of turbines, the first of which extracts kinetic energy from the expanding hot gases to power the compressor and the second of which extracts additional kinetic energy from the hot gases to power a fan adapted to generate the major portion of the thrust associated with the engine. After passing through the second turbine the hot gases are expelled from the engine, thereby generating the remaining portion of the thrust associated with the engine.
The overall efficiency of the gas turbine engine is heavily dependent upon the efficiency of the compressor. The pressure ratio of the compressor, that is to say the ratio of air pressure at the compressor outlet to air pressure at compressor inlet, is one of the significant parameters which determines the operating efficiency of the compressor. The higher the pressure ratio at a given compressor rotational speed, the greater the efficiency. The higher the air pressure at the outlet of the compressor, the greater the energy available to drive the turbines downstream of the compressor and hence to provide thrust generation by the engine.
In axial flow compressors, pressurization of air is accomplished in a multiplicity of compressor stages or sections, each stage being comprised of a rotating multi-bladed rotor and a nonrotating multi-vaned stator. Within each stage the airflow is accelerated by the rotor blades and decelerated by the stator vanes with a resulting rise in pressure. Each blade and vane has a precisely defined airfoil surface configuration or shape whereby the air flowing over the blade or vane is accelerated or decelerated respectively. The degree of air pressurization achieved across each blade-vane stage is directly and significantly related to the aforementioned precise airfoil surface shape.
It has been found that, in service, the surfaces of the compressor blades and vanes become coated with contaminants of various types. Oil and dirt from airfield runways have been found adhered to the blade and vane surfaces. Aluminum and other metal substances erode from the other portions, such as clearance seals, of the engine and are deposited on the blades and vanes. These surface contaminants alter the above-mentioned precise airfoil surface shape, disturbing the desired airflow over the blades and vanes and cause reduced pressure rises across the various compressor stages and hence a drop in compressor efficiency. Typically, the drop in efficiency results in reduced thrust output for a given engine speed. While thrust levels can be maintained by operating the engine at overspeed conditions, such operation results in increased engine maintenance and reduced engine life.
Removal of the aforedescribed contaminants from blades and vanes of in-service compressors is desirable to restore compressor and engine efficiency. Since it is both time-consuming and expensive to disassemble the engine from the aircraft and thence the compressor from the engine, it is also desirable to remove the aforedescribed contaminants while the engine is on-wing. Furthermore, any method utilized to remove the contaminants must not interfere with the structural or metallurgical integrity of other components of the engine. By way of example, an acceptable method must remove aluminum contaminants adhering to the blades and vanes of the compressor without deleteriously affecting other aluminum components of the engine. In this regard, it is known in the art that contaminants can be removed from the internal components of a gas turbine engine by ingesting, into the engine inlet at idle speed, substances generally characterized as liquid solvents. However, liquid solvents, because of their dispersive characteristics, chemically attack not only the contaminants but also other portions of the engine which are made of the same material as the contaminants. Hence, where contamination of the vanes and blades has resulted from material erosion of other engine components, the ingestion of liquid solvents into the engine has not proven to be an acceptable method of removing the contaminants.
Another known method of removing contaminants from the internal components of a gas turbine engine utilizes solid particle abrasives which are ingested into the engine at idle speeds. The abrasive particles impinge upon the contaminated surfaces dislodging the contaminants. However, materials used in the prior art as abrasives have proven to be unsatisfactory. More particularly, these abrasives have been found to be overly abrasive such that they not only dislodge the contaminants but also destroy the surface smoothness of the blade or vane. Furthermore, it is generally accepted that while most of the abrasive material will be ejected from the engine through the exhaust, some of the abrasive will remain in the engine. Prior art abrasives have either been noncombustible in which case the particles clog cooling holes of the turbine components and restrict needed cooling air flow or the abrasives are combustible but leave residue deposits which also clog turbine component cooling holes.
Applicant's novel invention addresses these and other insufficiencies associated with prior art methods by providing a new and useful method which includes the use of a material, the abrasive characteristics of which have been hitherto unrecognized and unapplied in the removal of contaminants.
Therefore, it is an object of the present invention to provide a method for removing contaminants from the surface of an article.
It is another object of the present invention to provide a method for removing contaminants from the internal components of an air-breathing machine such as a gas turbine engine and, more particularly, for removing contaminants from stator vanes and rotor blades associated with the compressor of such an engine.
It is still another object of the present invention to provide a method of removing contaminants from compressor stator vanes and rotor blades without deleteriously affecting the structural or metallurgical integrity of other portions of the gas turbine engine.
It is still another object of the present invention to provide a method of removing contaminants from compressor stator and rotor vanes wherein such method includes injecting an abrasive material into the engine inlet while the engine is operating at idle speed.
It is yet another object of the present invention to provide a method of removing contaminants from compressor rotor and stator blades wherein an abrasive material injected into a gas turbine engine will not, if burned in the hot sections of the engine, leave a residue sufficient to interfere with the proper operation of the engine.